U.S. patent number 4,606,943 [Application Number 06/674,486] was granted by the patent office on 1986-08-19 for method for preparation of semipermeable composite membrane.
This patent grant is currently assigned to Culligan International Company. Invention is credited to Stanley F. Rak, Kenneth Ward.
United States Patent |
4,606,943 |
Rak , et al. |
August 19, 1986 |
Method for preparation of semipermeable composite membrane
Abstract
An excellent reverse osmosis membrane having a high flux with
superior chlorine resistance and low salt passage can be obtained
by interfacially condensing a water soluble aromatic polyamide
prepolymer with an essentially monomeric, aromatic, amine reactive
polyfunctional acyl halide. The polyamide prepolymer may be
prepared through the condensation reaction of an aromatic diamine
and an aromatic anhydride acyl halide. Preferably the amide
prepolymer, prepared from metaphenylene diamine and trimelletic
anhydride acid chloride, is reacted with trimesoylchloride to form
the thin film membrane of the subject invention.
Inventors: |
Rak; Stanley F. (Mundelein,
IL), Ward; Kenneth (Wheeling, IL) |
Assignee: |
Culligan International Company
(Northbrook, IL)
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Family
ID: |
27045265 |
Appl.
No.: |
06/674,486 |
Filed: |
November 26, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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476727 |
Mar 18, 1983 |
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Current U.S.
Class: |
427/244;
210/500.28; 210/500.37; 264/41; 427/340; 427/342 |
Current CPC
Class: |
B01D
71/56 (20130101); B01D 69/125 (20130101) |
Current International
Class: |
B01D
71/56 (20060101); B01D 69/00 (20060101); B01D
71/00 (20060101); B01D 69/12 (20060101); B05D
003/10 (); B05D 005/00 () |
Field of
Search: |
;427/244,340,246,342
;210/500.2 ;264/41 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0015149 |
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Sep 1980 |
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EP |
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0056175 |
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Jul 1982 |
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EP |
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2027614 |
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Feb 1980 |
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GB |
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Other References
Odian, G., "Principles of Polymerization", second edition, New
York, John Wiley & Sons, 1981, pp. 152-154..
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Primary Examiner: Lawrence; Evan K.
Attorney, Agent or Firm: Gerstman; George H.
Parent Case Text
This is a continuation of application Ser. No. 476,727, filed Mar.
18, 1983, abandoned.
Claims
We claim:
1. The method for the preparation of a semipermeable composite
membrane which comprises:
reacting solutions of (1) a molar excess of an aromatic di-primary
amine with (2) a mixture of a major amount of trimellitic anhydride
acid halide and a minor amount of trimesoylchloride to form a water
soluble prepolymer; placing an aqueous solution of said prepolymer
on a porous support to form a prepolymer film; reacting said
prepolymer film with (3) a nonpolar solvent solution containing
trimesoylchloride; and drying the membrane formed thereby.
2. The method of claim 1 in which the aromatic di-primary amine is
in solution in a mixture of dichloromethane and dimethyl
formamide.
3. The method of claim 2 in which the ratio of dichloromethane to
dimethylformamide is 10 to 1.
4. The method of claim 1 in which ingredient (2) is in solution in
dichloromethane.
5. The method of claim 1 in which solution (3) contains heptane as
the nonpolar solvent and trimesoyl chloride at 0.5 percent
concentration.
6. The method of claim 1 in which said membrane is dried and cured
by heating at 130.degree. C. for 5 minutes.
7. The method of claim 1 in which ingredient (2) contains 9 molar
parts of trimellitic anhydride acid chloride per one molar part of
trimesoyl chloride.
8. The method of claim 1 in which said halide is chloride.
9. The method of claim 8 in which said solution containing
trimesoylchloride also contains a diacyl chloride.
10. The method of claim 9 in which said diacyl chloride is
isophthaloyl chloride.
11. The method for the preparation of a semipermeable, composite
membrane which comprises:
reacting solutions of (1) a molar excess of metaphenylene diamine
with (2) a mixture of about 9 molar parts of trimellitic anhydride
acid chloride with about one molar part of trimesoylchloride to
form a prepolymer;
placing an aqueous solution of said prepolymer on a porous support
to form a prepolymer film;
reacting said prepolymer film with (3) a nonpolar solvent solution
containing trimesoylchloride; and
drying the membrane formed thereby.
12. The method of claim 11 in which solution (3) contains heptane
as the nonpolar solvent and trimesoyl chloride at 0.5%
concentration.
13. The method of claim 11 in which said solution containing
trimesoylchloride also contains a diacyl chloride.
14. The method of claim 13 in which said diacyl chloride is
isophthaloyl chloride.
Description
The subject invention relates to a method for preparing a
semipermeable membrane, particularly for use in reverse osmosis
systems.
The removal of solutes from a solution by the separation of those
solutes from the carrier solvent through a system utilizing a
process known as reverse osmosis is well known in the art. Such a
system typically has a semipermeable barrier membrane separating
the solvent from the solution. The solution, usually aqueous, is
introduced into one compartment of the system through a pump at
pressures up to 1000 psig, the pressure being dependent chiefly on
the species and concentration of the solutes. Both purified solvent
and concentrated solution are continuously withdrawn from the
system.
The effectiveness and efficiency of reverse osmosis apparatus
depends principally on the performance of the membrane. In
applications involving the desalination of seawater or brackish
water sources, a reverse osmosis membrane must have high salt
rejection characteristics, be capable of a high flux rate, and be
resistant to deterioration by hydrolysis and by exposure to high
pressure, temperature and dissolved chlorine.
An efficient reverse osmosis process generally requires a salt
rejection capability of greater than 95%. Greater than 99.5% salt
rejection characteristics is preferred. With such a capability,
seawater of a typical 35,000 ppm salt content can, in a single pass
through the system, be reduced to potable water of 175 ppm, a
concentration much less than many untreated tap waters.
The flux rate or fluid flow rate through the membrane is important
to the economics of the operation. High membrane flux rates permit
the system to be built with less membrane and other associated
equipment.
Chlorine and other oxidizing agents are often present in the
solutions fed to a reverse osmosis system being utilized for
fighting bacteria growth and the like. The presence of chlorine in
the feed can greatly affect the life of the membrane through a
mechanism of degradation that has been postulated as a reaction
with primary amidic hydrogens. Such chemical degradation results in
a relatively short useful life of the membrane, drastically
reducing the ability of a membrane to reject salt over a relatively
short period of time.
The first practical membranes utilized in reverse-osmosis
procedures were formed of cellulose diacetate, being characterized
by a very thin, dense surface layer adjacent to a much thicker
supporting layer. Further development in this area introduced the
ultrathin film secured to a separate thicker porous support. While
initially prepared separately, the film or membrane can now be
formed in situ on the support layer by a technique known as
interfacial condensation. The history of this art as taught in the
scientific literature and patents may be found in U.S. Pat. No.
4,277,344. In addition, the above-identified patent provides
specific examples of this technique.
SUMMARY OF THE INVENTION
Therefore, an object of the subject invention is a method for
preparing an improved semipermeable membrane for use in reverse
osmosis systems.
Another object of the subject invention is a method for the
preparation of a semipermeable membrane which has excellent
salt-rejection characteristics, variably controlled flux rates,
resistance to biological and hydrolytic degradation, reduced pH
sensitivity, and improved resistance to deterioration in the
presence of chlorine-containing feed water.
These and other objects are provided by the subject invention
wherein an excellent reverse osmosis membrane can be obtained by
condensing a water soluble aromatic polyamide prepolymer with an
essentially monomeric, aromatic, amine reactive polyfunctional acyl
halide. The prepolymer may be prepared through the condensation
reaction of an aromatic diamine and an aromatic anhydride,
preferably the reaction between metaphenylene diamine and
trimellitic anhydride acid chloride. The reverse osmosis membrane
prepared by the method of the invention comprises a microporous
substrate, preferably of polysulfone supported by polyester
non-woven fabric, and an ultrathin film or membrane having
semi-permeable properties deposited or secured to one side of the
microporous substrate. The procedure for preparing the
above-described membrane includes the steps of (a) treating an
appropriate microporous substrate with an aqueous solution of the
previously prepared polyamide prepolymer; (b) contacting the
prepolymer coated substrate with a solution of an polyacyl halide
in a nonpolar solvent where an interfacial condensation reaction
occurs; and (c) heat curing the composite membrane.
DETAILED DESCRIPTION OF THE INVENTION
As will be described in greater detail below, composite reverse
osmosis membranes characterized by controlled flux, high rejection
of solutes, and good resistance to attack by chlorine can be
prepared by the interfacial polymerization reaction of a layer or
film of an aqueous solution of the amine prepolymer having terminal
primary amines on a porous support with, for example, a triacyl
halide in a nonpolar solvent, particularly as exemplified by a
solution of trimesoyl chloride, i.e., 1,3,5-benzenetricarboxylic
acid chloride in heptane. The amine prepolymer which may be used to
form the membrane of the subject invention may be prepared as set
forth below.
In the conduct of this interfacial reaction, the acyl halide groups
react with the primary amine groups of the prepolymer to produce
amide linkages. Reaction is essentially instantaneous at the
interface of polyacyl chlorides with amines. The three-pronged
functionality of the triacyl halides is theorized to lead to the
generation of a highly crosslinked, three-dimensional polymeric
network in the membrane. The reverse osmosis membrane material is
thus a polymer approaching a large molecular weight. While the
prior art has recognized that diacyl halides do not necessarily
improve the performance of the resulting membrane when used in
conjunction with the triacyl halides, they may be of use in
adjusting certain physical properties of the membrane such as
specific ion rejection, permeate flux, and the like. 1 to 1 through
10 to 1 ratios of triacyl halides to diacyl halides appear most
effective.
As a direct result of the high degree of crosslinking, the reverse
osmosis membrane of the subject invention is generally insoluble in
virtually any solvent that does not first seriously degrade its
molecular structure. However, not all of the acyl halide functional
groups become bound into amide linkages. A substantial proportion
of the acyl halide functional groups are hydrolyzed by the water
present in the amine reagent as solvent, generating carboxylic acid
groups or carboxylate salts. These carboxyl groups have been
discovered to exert surprising effects on the performance of the
interfacial membrane, in that they affect flux and profoundly
affect the membrane's rejection of aqueous dissolved solutes.
The amine prepolymer can be formed by the condensation reaction of
an aromatic diamine and an aromatic anhydride. Examples of aromatic
diamines suitable for use in the preparation of the amine
prepolymer are: ##STR1## where R=H, CH.sub.3, Halogen; and ##STR2##
where R.sub.1 =--O--; ##STR3##
Examples of the aromatic anhydride which may be used to prepare the
amine prepolymer are: ##STR4## where X=halogen group. If
X=chloride, the above compound is trimellitic anhydride acid
chloride. ##STR5## where R.sub.2 = ##STR6##
In addition, a polyacyl halide, such as trimesoyl chloride, or
isophthaloyl chloride may be added to the reaction mixture of amine
and anhydride to vary the properties of the resulting reverse
osmosis membrane. The addition of such an acyl chloride when
preparing the prepolymer would tend to add more crosslinking, which
can affect the processibility of the membrane of the subject
invention. Such addition of a strengthening crosslinking agent may
also have the effect of reducing flux, though any noticeable
consequence would depend greatly on the amount and identity of the
acyl chloride added. As a result, generalizations concerning the
effects of such additions cannot be reliably made.
In preparing the amine prepolymer, the aromatic diamine as set
forth above is dissolved in a solution of methylene chloride
(dichloromethane) and dimethyl formamide. The solution of the
aromatic anhydride in dichloromethane is filtered to remove any
hydrolyzed anhydride, and added to the amine solution with rapid
stirring. The resulting solution is filtered, and the precipitate
dried.
When meta-phenylenediamine and trimellitic anhydride acid chloride
are the respective reactants, the prepolymer thus prepared has an
average molecular weight in excess of approximately 400 and is
primary amine terminated. The molecular formula of such an amine
prepolymer can be represented as: ##STR7## where Ar represents any
carbocyclic monocyclic aromatic nucleus free of any acyl halide
reactive group other than terminal amine groups and n represents a
chain length from 1-10. It should be recognized that varying
concentrations of prepolymers of different chain lengths, may be
prepared dependent chiefly on the relative concentration of the
reactants and crosslinking substituents.
After forming the amine prepolymer, the thin film composite
membranes of the subject invention may be formed by a series of
steps comprising (1) application of an aqueous amine prepolymer
solution to the porous support; (2) reaction with the polyacyl
halide, by contacting the prepolymer containing support with the
polyacyl halide solution; and (3) curing by heating in an oven at
approximately 110.degree.-150.degree. C., preferably 130.degree.
C.
The porous support may be any of the type conventionally used in
reverse osmosis processes. The preferred supports, however, are
those prepared from organic polymeric materials such as
polysulfone, chlorinated polyvinyl chloride, polyvinyl butyral,
polystyrene, cellulose esters, etc. Polysulfone film has been found
to be a particularly effective support material for the membranes
of the invention. Such polysulfone supports can be prepared by
depositing a layer of polysulfone (Union Carbide P-3500) solution
on a polyester unwoven fabric support material.
To the aqueous amine prepolymer may be added an agent for lowering
its surface tension, i.e., increasing the wetting capability of the
aqueous amine prepolymer solution. Detergents, such as the salts of
alkyl hydrogen sulfates having a carbon chain length of C.sub.12 to
C.sub.18 are particularly desirable. Specifically, sodium lauryl
sulfate, n-C.sub.11 H.sub.23 CH.sub.2 OSO.sup.- Na.sup.+,
exemplifies that which may be used.
The polyacyl halide of choice is trimesoyl chloride, primarily
because of its ability to crosslink and form insoluble films.
However, other polyacyl halides, such as that presented by the
formula: Ar(COX).sub.a wherein Ar is a mono- or polynuclear
aromatic nucleus free of amine reactive substituents other than
(COX); X is halogen; and a.gtoreq.2. The polyacyl halide should be
at least 0.01 weight-% soluble in liquid C.sub.1 -C.sub.12 alkane
or liquid halogenated lower alkane solvents. The 0.01
weight-percent represents the lower limit of solubility of the
polyacyl halide in the nonpolar solvent which can be used in the
interfacial polymerization reaction; concomitantly, ease of
production on a commercial scale dictates a level of solubility of
at least 1 weight-percent or more of the polyacyl halide in a
suitable nonpolar solvent. Actually, most aromatic polyacyl halides
are readily soluble in liquid aliphatic solvents such as the
pentanes, hexanes, heptanes, octanes, etc. which are substantially
inert toward the preferred porous support materials such as the
polysulfones.
After formation of the ultrathin membrane by interfacial
condensation reaction of the amine prepolymer and polyacyl halide,
the composite is generally cured at 130.degree. C. for 5 minutes.
Other temperatures and times may be used to achieve the desired
cure.
In the Examples which follow, all parts and percentages are by
weight unless otherwise indicated.
EXAMPLE 1
To 500 ml of dichloromethane is added 25.0 g (0.24 moles) of
metaphenylene diamine (MPD) and 13.2 g (0.16 moles) of
dimethylformamide (DMF). To another 200 ml of dichloromethane, 16.0
g (0.08 moles) of trimellitic anhydride acid chloride (TMAAC) is
added, and after this in solution, it is filtered to remove
hydrolyzed TMAAC.
With rapid stirring of the MPD/DMF solution prepared above, slowly
(15-20 ml/min) add the filtered TMAAC solution. This reaction is
carried out at room temperature, but a slight increase in
temperature will be observed, and should not boil the CH.sub.2
Cl.sub.2 if slow addition of the TMAAC is observed.
After the addition is complete, immediately filter the reaction
solution. Wash the precipitated prepolymer with 500 ml of CH.sub.2
Cl.sub.2, and collect the precipitated again with suction. Dry the
prepolymer at 30.degree. C. under vacuum for 24 hours.
A polysulfone support film was prepared from a 15% solution of
Union Carbide's P-3500 polysulfone in DMF. Sixteen grams of the
amine prepolymer was dissolved in 0.5% NaOH solution with 0.1%
sodium lauryl sulfate added to form a 2% amine prepolymer solution.
The polysulfone support film was coated by immersion in the amine
prepolymer solution. Excess amine prepolymer solution was removed
by draining and the wet coated polysulfone film was immediately
covered with a 0.5% heptane solution of trimesoylchloride (TMC).
Contact time for the interfacial reaction was 10 seconds. The
resulting composite membrane was further cured by heating at
130.degree. C. for 5 minutes. The membrane was placed in a cell
designed for characterizing RO membrane films and at 200 PSI. The
membrane rejected 99.1% of the dissolved salt from a 2000 PPM
sodium chloride solution, and at a flux of 5 gallons per square
foot per day (GFD).
EXAMPLE 2
A composite membrane was made according to the procedure of Example
1, with the exception that no final curing step was employed. No
rejection of salt was observed in the subsequent test under the
conditions of Example 1.
EXAMPLE 3
The procedure of Example 1 was followed except the ratio of MPD to
TMAAC was increased to 4 to 1 and cured at 112.degree. C. for 5
minutes. The observed flux was 5.6 GFD with a salt rejection of
98.5%.
EXAMPLE 4
The procedure of Example 1 was followed except the ratio of MPD to
TMAAC in the prepolymer was increased to 5 to 1 and the membrane
was cured at 110.degree. C. for 5 minutes. The observed flux was
5.6 GFD with a 98.8% salt rejection.
EXAMPLE 5
The procedure of Example 1 was followed, however, to the triacyl
chloride was added sufficient diacyl chloride in the form of
isophathoyl chloride to achieve a ratio of (a) 7.5 to 1 and (b) 4.2
to 1. The observed flux was (a) 3.7 GFD and (b) 10.1 GFD; the salt
rejection for each was (a) 98.5% and (b) 85%.
EXAMPLE 6
The procedure of Example was followed, however in (c) the relative
volumetric amount of DMF and CH.sub.2 Cl.sub.2 was changed to a
volumetric ratio of 1 DMF/10 CH.sub.2 Cl.sub.2 in the prepolymer
reaction medium as opposed to 0.22/10 in (a) and (b). In addition,
1 mole of Trimesoylchloride (TMC) was added in preparing the
prepolymer for every 9 moles TMAAC in (c). The prepolymer treated
porous support was immersed in a solution of 0.5% TMC in heptane to
form the membranes for which the following values were
observed:
______________________________________ (a) (b) (c)
______________________________________ flux 4 GFD 8 GFD 11.2 GFD
salt rejection 96% 98.5% 98.6%
______________________________________
While the invention has been described with reference to a
preferred embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substitued for elements thereof without departing from the scope of
the invention. In addition, many modifications may be made to adapt
a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *